When deciding which size conductor can be used can the 75° column be used for sizing the breaker or is it just for derating? Asked in reference to an answer in another thread.

 

Instead of what? The 60[SUP]o[/SUP]C column or the 90[SUP]o[/SUP]C column? In one chart that I often reference the only reference for THHN is for 90[SUP]o[/SUP]C. That shows an ampacity of 55 for #8 AWG copper. The only reference for THWN is for 75[SUP]o[/SUP]C. That shows 50A for the same conductor.

For double-labeled THHN/THWN I'd go with the lower value.

 

I see that now. I don't know why I thought there was a listing in the 60° column. Thank you for clearing up my confusion.

 

Different insulation types have different maximum operating temperatures. Type T thermoplastic is limited to 60 degrees C., Type TH is limited to 75 degrees C. and Type THH is limited to 90 degrees C. There are various other insulation types and each will be listed in one of these three temperature columns. However, you must also consider the temperature rating of the connecter that the wire is attached. All connectors are rated for 60 degrees C., some connectors are rated for 75 degrees C (mostly connectors for higher amperage, 40 amperes and above) and no connectors in common usage are rated for 90 degrees C.

So, while there ARE some connectors rated at the higher temperatures the likelihood of those connecters being used in a DIY project is almost nil.
That makes the 90 degree C. insulation column for Ampacities useless except for derating purposes.

 

Yeah, what Furd said. NOT what Nashkat1 said.

It is just for derating, if any of the screw, lug or wirenut connectors are rated at 60 degrees. So, even though THWN is in the 75 degree column, the connector forces you to use the 60 degree column as the maximum limit for that circuit. Derating can still push you below that maximum.

If all connections are rated at or above 75 degrees, the maximum limit for a THWN circuit is found in the 75 degree column

Dual rated THHN/THWN. In a dry location the wire uses the THHN column. In a wet location the wire uses the THWN column. Dry and wet locations are defined in the code and don't necessarily follow common sense.

 

Just for refference my question refered to post #8in this thread: http://www.doityourself.com/forum/el...ml#post2081351

 

I'm curious. What did you hear me say?

 

Do you have a source for that?

If that is true, then these are also true:

1. The allowable ampacity for every type of conductor must be read from the 60[SUP]o[/SUP]C column in Table 310.15(B)(16), regardless of that conductor's ambient temperature rating; and
2. Both the 75[SUP]o[/SUP]C column and the 90[SUP]o[/SUP]C column in Table 310.15(B)(16) are useless for any practical purpose, since derating a conductor reduces its allowable ampacity.

I have never derated a conductor for any reason other than installing more than three current-carrying conductors in a single raceway. I've never seen anyone else do that either.

 

I don't have a copy of the current NEC but try 110.14 (C) This section is also referenced by the FPN at 315.15 (A) (2). (2002 NEC)


Not at all. As I previously stated, most devices that have a rating of 40 amperes or more DO have terminations rated at 75 degrees C. Some devices with a rating less than 40 degrees C. will also have the 75 degree C. rating although it isn't really relevant since conductor sizes 14, 12 and 10 are specifically limited to an Ampacity of 15, 20 and 30 respectively REGARDLESS of the insulation type used on the conductors.


Then you have never run conductors through a space that had an elevated temperature and/or you never connected to terminations with a 60 degree C. temperature rating where the conductor was sized to operated at the 75 degree C. rating. If you did, then you were in violation of the NEC.

 

Here is a Mike Holt video that explains it. I did make a mistake with the 40 ampere figure, it is 100 ampere.

https://www.youtube.com/watch?v=WL81zOzNagM

 

Hey, that's kinds cool. I find that I always enjoy hearing Mike Holt.

And I learned something. I learned where the rule is and why it exists. I knew that's how we did it in the field, but I didn't have the reference or context for it, and now I do. So that's cool too.

And I learned, as I said before, that the 75oC and 90oC columns in Table 310.15(B)(16) aren't there for derating purposes. They're there to use iff every piece of the system is rated for that temperature.

I've probably run conductors through a space that had an elevated temperature on some occasions. That's not a problem so long as the conductor is rated for that. I've also terminated conductors to devices rated at 60[SUP]o[/SUP]C, thousands of times. What I haven't done is decide to use a smaller conductor, based on the ampacity shown in the 75[SUP]o[/SUP]C or 90[SUP]o[/SUP]C column, to connect to one of those devices. So, no, I haven't done anything that would violate Section 110.14(C), so far as I know.

Thanks for the info. It's always good to have a direct reference.

 

Good info, Furd, thanks for posting it.

 

Actually they are for derating purposes. If you have more than three current-carrying conductors in a raceway then you must derate according to the schedule; four to six conductors is 80%, six to nine is 70% and so forth. But you must always keep in mind the terminals to which these wires terminate and any other rules.

For example, using #12 THHN and having nine current-carrying conductors you first look at the maximum Ampacity of #12 THHN when used in raceway or cable and it is 30 amperes. For nine current-carrying conductors you have to derate to 70% of the ultimate Ampacity or 24 amperes. HOWEVER, #12 conductors are limited to a maximum Ampacity of 20 amperes so that is the maximum Ampacity that can be used in this scenario. In effect, no derating from the normal 20 ampere Ampacity.

How about using 14 #12 conductors? If I remember correctly (I should look it up but...) 14 current-carrying conductors in a raceway require derating to 50% of maximum Ampacity. That would mean for the same THHN insulation that you could only load those conductors to a maximum of 15 amperes although the rating for #12 THHN is 20 amperes under most conditions. If you derated from the 75 degree C. column the Ampacity would be even less, only 12-1/2 amperes.

For #10 and smaller conductors the end result will always be the same or lower than shown in the 60 degree C. column but it can get quite interesting in the larger sizes. In these cases you absolutely must take into consideration the temperature rating of the terminal.